Structural changes in lignocellulosic biomass heated under conditions comparable to those encountered in several types of natural or planned burnings have been studied by solid-state 13C- and 15N-CPMAS NMR spectroscopy of 15N-enriched ryegrass (Lolium rigidum) after being subjected to progressive thermal treatment. The solid-state 15N-NMR spectra of biomass subjected to severe heating revealed amide-N in forms which are resistant to the thermal treatment. Progressive burning was found to occur in two well-defined stages: In the early stage the free amino acid and some NH2 groups were removed, but no substantial disruption of the peptide structure was observed. In the final stage of burning the amide-N was converted to heterocyclic structures such as pyrroles, imidazoles and indoles. Some evidence for the presence of pyridines and phenazines was also found at this stage. These findings suggest that a major portion of the N is in forms that may survive most natural fires and that their stability towards further microbial degradation is increased by the heating. The solid-state 13C-NMR spectra revealed that the carbohydrate fraction is converted into condensed dehydrated material producing intense signals in the aromatic region.